Optical phase modulation is utilized in a myriad of devices for application in spectroscopy, ellipsometry, displays, beam steering, diffractive optics spatial light modulation, switches, tunable filters and optical signal processing. Phase shifting can be provided by nematic liquid crystals upon application of an electric field. However, the response times are on the order of milliseconds. Chiral smectic liquid crystals (CSLCs) provide response times on the order of microseconds. However, in planar aligned cells (smectic layers oriented perpendicular to the substrate walls), application of an electric field perpendicular to the cell walls reorients the molecular directors in a plane parallel to the cell walls, providing electro-optic rotation of the optic axis of the cell, but not providing variable birefringence. Thus planar-aligned CSLCs cannot, on their own, provide analog phase modulation.
Phase modulation can be achieved with a planar-aligned CSLC quarter-half-quarter variable retarder, comprising a CSLC rotatable half-wave plate and two passive quarter-wave plates positioned in series with and on either side of the half-wave plate (Sharp et al. in U.S. patent application Ser. No. 792,284). For linearly polarized light the polarization is preserved and pure phase modulation is achieved when the quarter-wave plates are oriented at 45.degree. to the axis of polarization. For an orientation of the optic axis of the half-wave plate at an electro-optically rotatable angle .alpha. with respect to the polarization, the phase shift is 2.alpha..
Variable retardance can also be provided by CSLCs in the homeotropic alignment, wherein the smectic layers are parallel to the cell walls (Sharp et al. in U.S. patent application Ser. No. 792,284). Application of an electric field parallel to the cell walls rotates the molecular directors in a plane perpendicular to the cell walls. This provides variable retardance with a fixed orientation of the optic axis. In this respect, homeotropically aligned CSLCs are similar in function to nematic liquid crystals, but with at least two orders in magnitude shorter response times. Because of the use of lateral electrodes, the aperture of homeotropically aligned cells is limited.
One particular application of phase modulators is within a Fabry-Perot etalon. In a Fabry-Perot resonance cavity, transmission depends on satisfying the resonance condition that the round-trip phase delay equal an integral number of wavelengths of intracavity light. At resonance the reflected waves from each pass through the cavity interfere constructively and the light is transmitted. Tuning the phase delay within the cavity tunes the resonant wavelengths of light. A Fabry-Perot cavity requires two reflective surfaces. Typically these are ordinary mirrors comprised of dielectric stacks which add to the fabrication complexity and expense of the modulator.